1. Field of the Invention
The invention relates to an axial-flow turbomachine, particularly a gas turbine, having a plurality of vane carrier segments which are suspended coaxially in a machine casing and in which the guide vanes are held by their roots, the root of each guide vane engaging in each case in two vane carrier segments adjoining one another in the axial direction.
2. Discussion of Background
Axial-flow turbomachines are usually equipped with a vane carrier for the guide vanes which is suspended coaxially in a casing, while both the casing and the vane carrier are provided with a horizontal junction plane in which their respective top and bottom halves are bolted together.
In unsteady operation, such as frequently occurs in power station gas turbines in the covering of peak loads, thermal stresses and differential expansions arise in the casings and lead to deformations of the casings and to variations of clearance between casing and rotor. Thus, for example, in such thermal turbomachines the problem of the ovalization of the axially divided casings concerned is as old as the machines themselves. The causes of these undesirable deformations are to be seen mainly in the presence of temperature gradients over the thickness of the wall, which give rise to a difference in creep between the outer and inner fibers respectively, and in the asymmetry caused by the junction planes.
In highly loaded gas turbines, in which the vane carriers assume temperatures of 400.degree. to 450.degree. C. during operation despite being cooled, a variation of the cooling could possibly affect the ovalization. However, the radial expansion of the vane carrier could thereby be affected in such a manner that vane clearances are thereby reduced, which would lead to an increased risk of grazing.
Another measure consists in dispensing with the vane carrier in the form of half-shells bolted together and in suspending the guide vanes in vane carrier segments of the kind mentioned above. These vane carrier segments are in turn suspended in radial ribs of the casing. This solution is based on the fact that the thermal deformation of the outer casing is less than that of a half-shell vane carrier. The guide vanes are in this case held by their roots in the vane carrier segments, the root of each guide vane engaging in each case in two vane carrier segments adjoining one another in the axial direction. A disadvantage of this arrangement is that the faces of the vane carrier segments form the walls which limit the flow in the turbomachine and, as such, are exposed to the high temperatures prevailing in the flow duct. Consequently, the segments must be made of a high-grade material.